1. Field of the Invention
The present invention relates to a redundant system including at least one radio circuit, and in particular to a DCME (Digital Circuit Multiplication Equipment) system used for a radio circuit, wherein DCME is defined in ITU-T Recommendation G.763.
2. Description of the Related Art
In general, a communication system using a radio circuit has employed redundant configuration so that, when a failure occurs in a working circuit. it can be switched to another circuit to ensure communication reliability. In satellite communications systems. both circuits of the redundant system are a radio circuit. In terrestrial communications systems, both circuits are a radio circuit in many cases. A wired circuit may be used as the other circuit.
A communication system using DCME is typically formed in conformity with ITU-T Recommendations (G.704, G.732, G.763). The ITU-T G.704 stipulates provisions for frame structure in each bit rate. For example, for a 2048-kbps signal, it is provided to form each frame consisting of 32 time slots (TS). According to the provisions, a synchronization pattern is included in TSO, signaling is assigned to TS 16, and data and speech are assigned to other TSs, a multi-frame consists of 16 frames. and an insertion location of a CRC bit is also stipulated.
Furthermore, in ITU-T Recommendation G. 732, various circuit abnormalities (circuit alarms) including loss of a signal input, loss of a frame synchronizing signal, and a bit error, are stipulated. In ITU-T Recommendation G.763, various alarms (DCME alarms) concerning a DCME unit are stipulated. The DCME unit is defined as a digital circuit multiplexing equipment having both a speech compression function using ADPCM and a DSI (Digital Speech Interpolation) function. Use of these functions is recommended in order to increase the use efficiency of a radio circuit.
As for circuit abnormalities, there is stipulated such a scheme that a station which has detected an alarm issues a switching request to the opposite station, confirms a switching response supplied from the opposite station, and then they execute circuit switching.
In a satellite communication circuit, however, round trip propagation delay time is as long as 0.5 sec. If switching is conducted after waiting a switching response from the opposite station, therefore, rapidity is lost and the system reliability is hampered. Especially when a DCME unit stipulated In ITU-T Recommendation G.763 is used, it is necessary to detect both the circuit alarms stipulated in ITU-T Recommendation G.732 and ITU-T Recommendation G.763. This poses a problem that it takes more and more time to conduct switching.
A conventional circuit switching method in a redundant system has been disclosed In Japanese Patent Unexamined Publication No. 6-104859. According to this conventional method, each of communication devices connected through working and protection transmission lines stores switching condition information, which is used to automatically switch from the working transmission line to the other protection transmission line in case of occurrence of a failure on the working transmission line.
In satellite communications systems, however, a radio communication circuit is frequently in an unstable state hovering between the abnormal state and the normal state. In such an unstable state, the redundant system employing the above conventional switching method is caused to frequently switch between two transmission lines.
An object of the present invention is to provide a circuit switching method and a DCME system allowing rapid and reliable circuit switching operation.
According to the present invention, a DCME system for use in a redundant communication system having a plurality of circuits including at least one wireless circuit, includes a DCME unit provided for a single wireless circuit, wherein the DCME unit conforms to ITU-T Recommendation G.763 and a storage previously storing alarm conditions defined in ITU-T Recommendations G.732 and G.763. The DCME system further includes an alarm detector for detecting received alarm information from a received signal on each of the plurality of circuits and a determiner for determining whether a failure has occurred on each of the plurality of circuits, by comparing the alarm conditions read from the storage with the received alarm information detected by the alarm detector. A circuit selector selects one of the plurality of circuits depending on a determination result obtained by the determiner.
According to an aspect of the present invention, a DCME system for use in a redundant communication system having first and second radio circuits includes a DCME unit provided for each of the first and second radio circuits, wherein the DCME unit conforms to ITU-T Recommendation G.763 and a storage previously storing alarm conditions defined In ITU-T Recommendations G.732 and G.763. the DCME system further includes a first alarm detector for detecting first alarm information defined in ITU-T Recommendations G.732 and G.763 from a first received signal on the first radio circuit, and a second alarm detector for detecting second alarm information defined in ITU-T Recommendations G.732 and G.763 from a second received signal on the second radio circuit. The DCME system further includes a first determiner for determining whether a failure has occurred on the first radio circuit to produce a first determination result, by comparing the alarm conditions read from the storage with the first alarm information, and a second determiner for determining whether a failure has occurred on the second radio circuit to produce a second determination result, by comparing the alarm conditions read from the storage with the second alarm information. A circuit selector selects one of the first and second radio circuits based on the first and second determination results.
According to the first aspect of the present invention, attention is paid to the fact that the circuit switching can be conducted by using a switching request issued by the opposite station as a trigger. Thus, the DCME system is provided with a circuit switching function capable of determining whether a circuit failure occurs to the extent that switching should be executed, on the basis of the switching request (alarm information) issued by the opposite station and alarm information detected in the own station, and taking an appropriate action. In the own station, therefore, rapid circuit switching can be executed and communication circuit interruption can be prevented.
The first determiner may produce the first determination result which indicates one of a first alarm-on period of time during which a failure has occurred on the first radio circuit within a predetermined time period and a first alarm-off period of time during which a failure has not occurred on the first radio circuit within the predetermined time period. The second determiner may produce the second determination result which indicates one of a second alarm-on period of time during which a failure has occurred on the second radio circuit within the predetermined time period and a second alarm-off period of time during which a failure has not occurred on the second radio circuit within the predetermined time period.
Since the first determination result indicates one of a first alarm-on period of time and a first alarm-off period of time and the second determination result indicates one of a second alarm-on period of time and a second alarm-off period of time, it is possible to determine whether a failure occurs on the first radio circuit and the second radio circuit to the extent that circuit switching is performed from one radio circuit to the other radio circuit before the circuit switching operation. Therefore, the circuit switching can be prevented when the other radio circuit to which a working circuit switches continuously changes in alarm state.
According to a second aspect of the present invention, a DCME system for use in a redundant communication system having a wireless circuit and a wired circuit, includes a DCME unit provided for the wireless circuit, wherein the DCME unit conforms to ITU-T Recommendation G.763; a storage previously storing alarm conditions defined in ITU-T Recommendations G.732 and G.763; a first alarm detector for detecting first alarm information defined in ITU-T Recommendations G.732 and G.763 from a first received signal on the wireless circuit; a second alarm detector for detecting second alarm information defined in ITU-T Recommendations G.732 from a second received signal on the wired circuit; a first determiner for determining whether a failure has occurred on the wireless circuit to produce a first determination result, by comparing the alarm conditions read from the storage with the first alarm information; a second determiner for determining whether a failure has occurred on the wired circuit to produce a second determination result, by comparing the alarm conditions read from the storage with the second alarm information; and a circuit selector for selecting one of the wireless and wired circuits based on the first and second determination results.
According to the second aspect of the present invention, the DCME system for use In such a communication system including a wireless circuit and a wired circuit monitors the circuit states of the wireless circuit on the basis of alarm conditions stipulated in ITU-T G.732 and ITU-T G.763 and the received alarm information stipulated in ITU-T G.732 and ITU-T G.763 and detected from received signals of the wireless circuit, and take an action to switch the operational circuit from the wireless circuit to the wired circuit or maintain the working circuit as it is.
In the own station, therefore, rapid circuit switching can be executed and communication circuit interruption can be prevented in the same way as the first aspect of the present invention.